Weight paint subtraction yields strange results
Weight painting on one vertex group will affect others vertex groups only if the "auto normalize" and/or "Multi-paint" buttons are checked. 

The first option makes sure that the sum of all weights of every vertex is 1.0 (normalization); this operation takes over anyway, when Blender calculates the deformations, so it is unnecessary to activate it, it's useful only to have a visual rappresentation of the influences that weights will have on the mesh (i.e. if a vertex has weight 0.1 to one bone and 0.4 to another, when animating they are automatically treated as if they were 0.2 and 0.8; the auto-normalize function lets you have a visual rappresenattion of this behaviour, but it can be misleading during the painting process, as it has happened to you).

The second option works when two or more bones are selected, and lets you altre the weights values leaving the proportions between the selected bones unaltered.

In my opinion both options should be unchecked unless you really need them.